An air Traffic alert and Collision Avoidance System (TCAS) is an independent airborne system designed to act as a backup to Air Traffic Control (ATC) and the "see and avoid" tactic exercised by the flight crew of an airplane. After two decades of development by the Federal Aviation Administration (FAA) and private contractors, TCAS has matured to a level where United States public law now requires that a TCAS be installed on commercial airplanes with more than thirty seats, starting in December of 1990.
A TCAS-equipped airplane is surrounded by TCAS-protected airspace whose physical dimensions vary as a function of altitude and closure rate, i.e., the rate at which other airplanes are approaching the TCAS-equipped airplane. Being a time-based avionic system, TCAS continuously estimates and updates the flight paths of other airplanes through the interrogation of, and replies from, airborne radar beacon transponders located onboard the other airplanes. An airplane whose estimated flight path is projected to penetrate the TCAS-protected airspace is considered a collison threat (intruder) and annunciated to the flight crew of the TCAS-equipped airplane.
TCAS-protected airspace can be divided into a caution area and a warning area, based on the estimated time to the Closest Point of Approach (CPA). About 40-45 seconds prior to CPA an intruder penetrates the caution area and causes the annunciation of a Traffic Advisory (TA). If the intruder continues to come closer to the TCAS-equipped airplane, at about 20-25 seconds to CPA, the intruder reaches the warning area, resulting in the annunciation of a Resolution Advisory (RA). Both TAs and RAs are constantly updated and, therefore, provide real time position and advisory information.
TAs and RAs are annunciated both visually and aurally. The aural portion consists of voice messages. The visual portion of TA and RA annunciators includes a traffic display in the horizontal plane and, for RA annunciators, a resolution display in the vertical plane.
Depending on the nature of the avionics of a TCAS-equipped airplane, intruder positions are displayed and, when required, maneuver guidance is given--using shape, color and size cues for different levels of alert--on one or more of the following types of instruments: Electronic Horizontal Situation Indicator (EHSI), Electronic Attitude Director Indicator (EADI), Navigation Display (ND), Primary Flight Display (PFD), designated Weather Radar (WXR) or a modified (liquid crystal) Vertical Speed Indicator (VSI). The flight crew uses the displayed information as an aid to visually locating an intruder and, if necessary, taking action to avoid a collision.
In the past, resolution advisories, i.e., RAs, of TCAS II-equipped airplanes (TCAS II is the second generation of TCAS) have been displayed as a lighted red arc (with or without a green segment) on each pilot's modified VSI, now called an RA/VSI instrument. The lighted red arc forms either a corrective advisory, which calls for a change in the vertical speed of the TCAS II-equipped airplane, such that the vertical speed pointer of the VSI moves outside of the red arc, or a preventive advisory, which calls for restricted changes in the vertical speed of the TCAS II-equipped airplane, such that the vertical speed pointer remains outside of the red arc. The flight crew of the TCAS II-equipped airplane adjusts the vertical speed (rate of climb or descent) of the airplane in the manner dictated by the nature of the advisory. The change in vertical speed changes the flight path of the aircraft in a manner designed to gain sufficient separation from the intruder to eliminate the collision threat.
More specifically, existing TCAS II VSI RAs comprise a red arc (also called a red band) on the dial of each pilot's vertical speed indicator (VSI). The red arc indicates the region of vertical speed (climb or descent) to avoid. A green arc (also called a green band) that appears at the end of red arc corrective RAs is used to assist a pilot in determining which direction to maneuver and to suggest a range of vertical speed that is sufficient to avoid the red region without being excessive. RAs comprise either an Up Advisory or a Down Advisory. An Up Advisory is a red arc that emanates from -6,000 feet per minute (FPM) up to the vertical speed above which a collision threat is eliminated. A Down Advisory is a red arc that emanates from +6,000 FPM down to the vertical speed below which a collision threat is eliminated. Up and Down Advisories can occur simultaneously. In this instance two red arcs emanate up from -6,000 FPM and down from +6,000 FPM occur, to leave a region in between that indicates a vertical speed range adequate for traffic separation.
While TCAS II VSI RAs are satisfactory, they have certain disadvantages. They are not integrated into the primary electronic flight instrument systems of airplanes. They display control information on an instrument that is not used by a pilot to extract control information. They require an expensive retrofit for airplanes equipped with conventional VSIs. More importantly, they are not intuitive. More specifically, changing the climb or descent flight path of an airplane involves changing the pitch attitude of the airplane. Because pilots control pitch with reference to the attitude indicator of an airplane rather than the vertical speed indicator, a TCAS RA in the form of a pitch attitude advisory is more useful than is a VSI advisory.
The present invention is directed to providing a Resolution Advisory (RA) in the form of a pitch guidance symbol on the electronic attitude or primary flight display of an airplane instead of, or in addition to, a conventional TCAS II VSI RA.